全文获取类型
收费全文 | 5156篇 |
免费 | 213篇 |
国内免费 | 1篇 |
专业分类
5370篇 |
出版年
2023年 | 17篇 |
2022年 | 21篇 |
2021年 | 40篇 |
2020年 | 23篇 |
2019年 | 41篇 |
2018年 | 89篇 |
2017年 | 93篇 |
2016年 | 109篇 |
2015年 | 97篇 |
2014年 | 133篇 |
2013年 | 218篇 |
2012年 | 450篇 |
2011年 | 650篇 |
2010年 | 294篇 |
2009年 | 90篇 |
2008年 | 434篇 |
2007年 | 401篇 |
2006年 | 345篇 |
2005年 | 312篇 |
2004年 | 253篇 |
2003年 | 239篇 |
2002年 | 183篇 |
2001年 | 71篇 |
2000年 | 149篇 |
1999年 | 79篇 |
1998年 | 49篇 |
1997年 | 14篇 |
1996年 | 12篇 |
1995年 | 18篇 |
1994年 | 12篇 |
1993年 | 11篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1987年 | 18篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1982年 | 16篇 |
1981年 | 9篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 13篇 |
1977年 | 9篇 |
1976年 | 17篇 |
1975年 | 12篇 |
1974年 | 18篇 |
1973年 | 9篇 |
1972年 | 9篇 |
1971年 | 10篇 |
排序方式: 共有5370条查询结果,搜索用时 15 毫秒
81.
Abstract Viscum album L. ssp. album is semi-parasitic on deciduous trees and shrubs. In order to identify hosts and map the distribution of V. album ssp. album in Croatia and Slovenia, field research was carried out, and herbaria were surveyed. In Croatia and Slovenia, V. album ssp. album occurred on 59 taxa. In Croatia, there were 52 hosts (33 autochthonous and 15 allochthonous species, two cultivars and two hybrids). In Slovenia, there were 25 hosts (21 autochthonous and four allochthonous species). There were 18 hosts common to both countries, 34 hosts were found only in Croatia, and seven hosts only in Slovenia. The hosts belonged to 13 families. The majority of these (19 species) belong to the Rosaceae, followed by Salicaceae, Aceraceae, Betulaceae, Fagaceae, Juglandaceae, Tiliaceae, Hippocastanaceae, Ulmaceae, Oleaceae, Fabaceae, Moraceae and Viscaceae. All hosts have been previously recorded in the literature, except Alnus japonica (Thunb.) Steud., Amelanchier lamarckii F.G. Schroed. and Crataegus nigra Waldst. et Kit. The distribution of this mistletoe was scattered, due to the scattered distribution of hosts, local conditions, movement of bird-vectors, etc. A continuous distribution was found only in part of the distribution area of narrow-leaved ash (Fraxinus angustifolia Vahl). 相似文献
82.
Robert Brus Dalibor Ballian Faruk Bogunić Martin Bobinac Marilena IdžOjtić 《Plant biosystems》2013,147(2):278-285
Abstract In most European countries, the service tree (Sorbus domestica L.) is a rare and threatened species and its conservation has been recognised as a priority. The aim of this study was to asses its morphologic variation in the western and central part of the Balkan Peninsula and in southern Central Europe. Three populations were analysed: one in Serbia, one in Bosnia and Herzegovina and one in Slovenia. In each population 30 trees were selected, and from each tree 30 leaves were collected for morphometric analysis based on nine leaflet morphological traits. Univariate (ANOVA) and multivariate (MANOVA) analysis of variance were used to estimate the variation within- and between populations and a discriminant analysis was performed to examine the structure of the between-population differences. The values of particular morphological traits found in our study did not differ considerably from the values reported elsewhere. The results revealed significant within- and between population variation. Variation within populations was highly significant for all the scored leaf morphological traits, while variation between populations was significant for all the studied traits except for the leaflet length. The discrimination between the three populations was significant. High percentages of correctly classified samples demonstrate good discriminating employability of the analysed leaf morphological traits and indicate differentiation of the analysed populations. 相似文献
83.
Lorena Esposito-Bauer Tobias Saam Iman Ghodrati Jaroslav Pelisek Peter Heider Matthias Bauer Petra Wolf Angelina Bockelbrink Regina Feurer Dominik Sepp Claudia Winkler Peter Zepper Tobias Boeckh-Behrens Matthias Riemenschneider Bernhard Hemmer Holger Poppert 《PloS one》2013,8(7)
Purpose
The aim of this study was to investigate prospectively whether MRI plaque imaging can identify patients with asymptomatic carotid artery stenosis who have an increased risk for future cerebral events. MRI plaque imaging allows categorization of carotid stenosis into different lesion types (I–VIII). Within these lesion types, lesion types IV–V and VI are regarded as rupture-prone plaques, whereas the other lesion types represent stable ones.Methods
Eighty-three consecutive patients (45 male (54.2%); age 54–88 years (mean 73.2 years)) presenting with an asymptomatic carotid stenosis of 50–99% according to ECST-criteria were recruited. Patients were imaged with a 1.5-T scanner. T1-, T2-, time-of-flight-, and proton-density weighted studies were performed. The carotid plaques were classified as lesion type I–VIII. Clinical endpoints were ischemic stroke, TIA or amaurosis fugax. Survival analysis and log rank test were used to ascertain statistical significance.Results
Six out of 83 patients (7.2%) were excluded: 4 patients had insufficient MR image quality; 1 patient was lost-to-follow-up; 1 patient died shortly after the baseline MRI plaque imaging. The following results were obtained by analyzing the remaining 77 patients. The mean time of follow-up was 41.1 months.During follow-up, n = 9 (11.7%) ipsilateral ischemic cerebrovascular events occurred. Only patients presenting with the high-risk lesion types IV–V and VI developed an ipsilateral cerebrovascular event versus none of the patients presenting with the stable lesion types III, VII, and VIII (n = 9 (11.7%) vs. n = 0 (0%) during follow-up). Event-free survival was higher among patients with the MRI-defined stable lesion types (III, VII, and VIII) than in patients with the high-risk lesion types (IV–V and VI) (log rank test P<0.0001).Conclusions
MRI plaque imaging has the potential to identify patients with asymptomatic carotid stenosis who are particularly at risk of developing future cerebral ischemia. MRI could improve selection criteria for invasive therapy in the future. 相似文献84.
Martina Grdiša Sandra Babić Martina Periša Klaudija Carović‐Stanko Ivan Kolak Zlatko Liber Marija Jug‐Dujaković Zlatko Satovic 《化学与生物多样性》2013,10(3):460-472
Dalmatian pyrethrum (Tanacetum cinerariifolium (Trevir. ) Sch.Bip. ) is a plant species endemic to the east Adriatic coast. The bioactive substance of Dalmatian pyrethrum is a natural insecticide, pyrethrin, a mixture of six active components (pyrethrins I and II, cinerins I and II, and jasmolins I and II). The insecticidal potential of pyrethrin was recognized decades ago, and dried and ground flowers have traditionally been used in Croatian agriculture and households. A total of 25 Dalmatian pyrethrum populations from Croatia were studied to determine the pyrethrin content and composition, and to identify distinct chemotypes. The total pyrethrin content ranged from 0.36 to 1.30% (dry flower weight; DW) and the pyrethrin I/pyrethrin II ratio ranged from 0.64 to 3.33%. The statistical analyses revealed that the correlations between the percentage of pyrethrin I and of all the other components were significant and negative. The total pyrethrin content was positively correlated with the percentage of pyrethrin I and negatively correlated with cinerin II. The multivariate analysis of the chemical variability enabled the identification of five chemotypes among 25 Dalmatian pyrethrum populations. The chemical characterization of indigenous Dalmatian pyrethrum populations may serve as a good background for future breeding and agricultural exploitation. 相似文献
85.
Renata Jurišić Grubešić Goran Srečnik Dario Kremer Jadranka Vuković Rodríguez Toni Nikolić Sanda Vladimir‐Knežević 《化学与生物多样性》2013,10(7):1305-1316
A rapid reversed‐phase (RP) high‐performance liquid chromatography method was developed and applied for simultaneous separation, and determination of flavonoids and phenolic acids in eight Plantago L. taxa (P. altissima L., P. argentea Chaix , P. coronopus L., P. holosteum Scop . ssp. depauperata Pilger , P. holosteum ssp. holosteum, P. holosteum ssp. scopulorum (Degen) Horvati? , P. lagopus L., and P. maritima L.) growing in Croatia. Chromatographic separation was carried out on Zorbax Eclipse XDB‐C18 using gradient elution with a H2O (pH 2.5, adjusted with CF3COOH) and MeCN mixture at 30°. The contents of analyzed phenolic compounds (% of the dry weight of the leaves, dw) varied among examined species: rutin (max. 0.024%, P. argentea), hyperoside (max. 0.020%, P. lagopus), quercitrin (max. 0.013%, P. holosteum ssp. holosteum), quercetin (max. 0.028%, P. holosteum ssp. scopulorum), chlorogenic acid (max. 0.115%, P. lagopus), and caffeic acid (max. 0.046%, P. coronopus). Isoquercitrin was detected only in P. argentea (0.020%), while isochlorogenic acid content was below limit of quantification in all investigated species. Multivariate analyses (UPGMA and PCA) showed significant differences in contents of investigated polyphenolic compounds between different Plantago taxa. Accordingly, investigated substances might be employed as chemotaxonomic markers in the study of the complex genus Plantago. 相似文献
86.
Nemanja Rajčević Peđa Janaćković Srđan Bojović Vele Tešević Petar D. Marin 《化学与生物多样性》2013,10(1):144-156
The essential‐oil compositions of one Croatian and three Serbian populations of Juniperus deltoides R.P.Adams have been determined by GC/MS analysis. In total, 147 compounds were identified, representing 97.3–98.3% of the oil composition. The oils were dominated by monoterpenes, which are characteristic components for the species of the section Juniperus. Two monoterpenes, α‐pinene and limonene, were the dominant constituents, with a summed‐up average content of 49.45%. Statistical methods were used to determine the diversity of the terpene classes and the common terpenes between the newly described J. deltoides populations from Serbia and Croatia. Only reports on several specimens from this species have been reported so far, and there are no studies that treat population diversity. Cluster analysis of the oil contents of 21 terpenes showed high correlation with the geographical distribution of the populations, separating the Croatian from the Serbian populations. The comparison of the essential‐oil compositions obtained in the present study with literature data, showed the separation of J. deltoides and J. oxycedrus ssp. oxycedrus from the western Mediterranean region. 相似文献
87.
88.
Ana Vrsalović Presečki Zvjezdana Findrik Blažević Đurđa Vasić-Rački 《Bioprocess and biosystems engineering》2013,36(1):117-126
The first step of starch hydrolysis, i.e. liquefaction has been studied in this work. Two commercial α-amylases from Bacilllus licheniformis, known as Termamyl and Liquozyme have been used for this purpose. Using starch as the substrate, kinetics of both enzymes has been determined at optimal pH and temperature (pH 7, T = 80 °C) and at 65 °C and pH 5.5. Michaelis–Menten model with uncompetitive product inhibition was used to describe enzyme kinetics. Mathematical models were developed and validated in the repetitive batch and fed-batch reactor. Enzyme inactivation was described by the two-step inactivation model. All experiments were performed with and without calcium ions. The activities of both tested amylases are approximately one hundred times higher at 80 °C than at 65 °C. Lower inactivation rates of enzymes were noticed in the experiments performed at 65 °C without the addition of calcium than in the experiments at 80 °C. Calcium ions in the reaction medium significantly enhance amylase stability at 80 °C and pH 7. At other process conditions (65 °C and pH 5.5) a weaker calcium stabilizing effect was detected. 相似文献
89.
Interaction of Insulin-Like Growth Factor-Binding Protein 2 with α2-Macroglobulin in the Circulation
Insulin-like growth factors (IGFs) play active role in mitogenic and metabolic processes. In the peripheral circulation, they are mostly bound to specific IGF-binding proteins (IGFBPs). Proteolysis of IGFBPs releases free, active IGFs. IGFBP-2 is the second most abundant of the six binding proteins and its concentration increases in catabolic states. The possible interaction between IGFBP-2 and other proteins in the circulation was investigated in this study. Our results showed that IGFBP-2 associates with α2-macroglobulin (α2M), a protease inhibitor. Formation of IGFBP-2/α2M complexes most likely contributes to the regulation of IGFBP-2 proteolysis and, thus, the activity of IGFs. 相似文献
90.
Felix Moerman Angelina Arquint Stefanie Merkli Andreas Wagner Florian Altermatt Emanuel A. Fronhofer 《Evolution; international journal of organic evolution》2020,74(3):573-586
Abiotic stress is a major force of selection that organisms are constantly facing. While the evolutionary effects of various stressors have been broadly studied, it is only more recently that the relevance of interactions between evolution and underlying ecological conditions, that is, eco-evolutionary feedbacks, have been highlighted. Here, we experimentally investigated how populations adapt to pH-stress under high population densities. Using the protist species Tetrahymena thermophila, we studied how four different genotypes evolved in response to stressfully low pH conditions and high population densities. We found that genotypes underwent evolutionary changes, some shifting up and others shifting down their intrinsic rates of increase (r0). Overall, evolution at low pH led to the convergence of r0 and intraspecific competitive ability (α) across the four genotypes. Given the strong correlation between r0 and α, we argue that this convergence was a consequence of selection for increased density-dependent fitness at low pH under the experienced high density conditions. Increased density-dependent fitness was either attained through increase in r0, or decrease of α, depending on the genetic background. In conclusion, we show that demography can influence the direction of evolution under abiotic stress. 相似文献